This invention relates to matching P wave and S wave seismograms and more particularly, to determining the ratio of the velocities of compressional and shear waves in seismic exploration.
Seismic energy propagates in a compressional, or P wave, and in a shear, or S wave, mode. Compressional and shear waves travel at different velocities through the earth and ratio of these velocities, V.sub.p /V.sub.s, is a characteristic of the formations through which the waves propagate.
The ratio of the velocities of the P waves and the S waves indicates the composition of the subsurface layers if sufficiently accurate measurements of the V.sub.p /V.sub.s ratio can be made. See Gregory, A. R., "Fluid Saturation Effects on Dynamic Elastic Properties of Sedimentary Rocks", Geophysics, Vol. 41, No. 5, pp. 895-921, October, 1976; and Pickett, G. R., "Acoustic Character Logs and Their Application to Formation Evaluation", Journal of Petroleum Technology, June, 1963.
Frequently, it is important for the interpreter to consider a P wave and an S wave seismic section together. In order to do this, a match (usually referred to as a correlation) must be made between the same points on the two sections. It is necessary to locate the same coal seam, or gas pocket, for example, on both sections. This matching (correlation) has generally, in the past, been performed by interpreters in a time consuming manner. Further, the results are usually unsubstantiated unless actual well logs are available to aid in the correlation.
It is an object of the present invention to automatically match, or correlate, events in P wave and S wave seismic sections.
One way of performing this correlation would be to directly map every seismic event from one section to the other. Using starting points on the P wave and S wave traces, each deviation on the P wave trace would be matched with one on the S wave trace by stretching or compressing the S wave trace to fit. This assumes that the seismograms are ideal. In reality, sizable events may be recorded on one trace that are small or completely missing on the other. Because of this, the automated matching of S wave and P wave sections has in the past been computer intensive and subject to becoming "lost" in portions of the traces wherein seismic events appear on one trace, but not in the other.